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Date: Tue, 5 May 2020 20:01:31 -0400 From: "Michael S. Tsirkin" <mst@...hat.com> To: linux-kernel@...r.kernel.org Cc: Thomas Gleixner <tglx@...utronix.de>, Eric Dumazet <eric.dumazet@...il.com>, Jason Wang <jasowang@...hat.com>, "David S. Miller" <davem@...emloft.net>, virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org Subject: [PATCH] virtio_net: fix lockdep warning on 32 bit When we fill up a receive VQ, try_fill_recv currently tries to count kicks using a 64 bit stats counter. Turns out, on a 32 bit kernel that uses a seqcount. sequence counts are "lock" constructs where you need to make sure that writers are serialized. In turn, this means that we mustn't run two try_fill_recv concurrently. Which of course we don't. We do run try_fill_recv sometimes from a fully preemptible context and sometimes from a softirq (napi) context. However, when it comes to the seqcount, lockdep is trying to enforce the rule that the same lock isn't accessed from preemptible and softirq context. This causes a false-positive warning: WARNING: inconsistent lock state ... inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage. As a work around, shut down the warning by switching to u64_stats_update_begin_irqsave - that works by disabling interrupts on 32 bit only, is a NOP on 64 bit. Reported-by: Thomas Gleixner <tglx@...utronix.de> Suggested-by: Eric Dumazet <eric.dumazet@...il.com> Signed-off-by: Michael S. Tsirkin <mst@...hat.com> --- I'm not thrilled about this but this seems the best we can do for now. Completely untested. Thomas, can you pls let me know the config I need to trigger the warning in question? drivers/net/virtio_net.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index 6594aab4910e..95393b61187f 100644 --- a/drivers/net/virtio_net.c +++ b/drivers/net/virtio_net.c @@ -1243,9 +1243,11 @@ static bool try_fill_recv(struct virtnet_info *vi, struct receive_queue *rq, break; } while (rq->vq->num_free); if (virtqueue_kick_prepare(rq->vq) && virtqueue_notify(rq->vq)) { - u64_stats_update_begin(&rq->stats.syncp); + unsigned long flags; + + flags = u64_stats_update_begin_irqsave(&rq->stats.syncp); rq->stats.kicks++; - u64_stats_update_end(&rq->stats.syncp); + u64_stats_update_end_irqrestore(&rq->stats.syncp); } return !oom; -- MST
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